Vapor Deposition (to be referred to as "CVD" hereinafter) is a widely-used method for depositing a thin film on a substrate. CVD has been extensively described in the literature, including in the patent literature, and has been comprehensively reviewed by C. E. Morosanu in "Thin Films by Chemical Vapor Deposition", Elsevier, New York (1990).
In CVD, a heat decomposable volatile compound (often an organometallic compound), which may be called the precursor, is contacted with a substrate which has been heated to a temperature above the decomposition temperature of the compound. A coating forms on the substrate which may be a metal, metal mixture or alloy, ceramic, metal compound or mixture thereof and the like, depending on the choice of precursors and reaction conditions.
The desirable characteristics of CVD as a thin film formation method can include its ability to produce a thin film with good step coverage on a substrate having projections, the ability to readily control the composition of the thin film, and the ability to form a thin film without contamination of, or damage to, the substrate.
The deposition of metals from the vapor phase is important in many industries, including the electronics industry. In this industry, metallic depositions are often undertaken involving metals such as aluminum, copper, silver, gold, and tungsten. In particular, these metals are often used for interconnection lines for semiconductor chips, circuits, and packages. For microelectronic applications, it is often desirable to deposit films having high conductivity, which typically means that the films must have minimal carbon and oxygen contamination.
In the electronics and other industries, there is a growing need for volatile sources of different metals to be used in the CVD of metallic films, metal oxide films, metal silicide films, and the like. The key property required for such metal sources is that they readily evaporate or sublime to give a metal-containing vapor or gas, with or without the use of an additional carrier gas, which vapor or gas can be decomposed in a controlled manner to deposit a film onto a target substrate.
Another use of CVD is to deposit films of metals into vias, trenches, and other recesses or stepped structures. Since it is the situation that the deposition must often occur onto substrates which have irregular topography, a technique is needed to provide conformal deposition, i.e., deposition of continuous layers over irregular substrates. When conformal thin-film deposition is required, techniques such as evaporation and sputtering (which are line-of-sight techniques) cannot be used. Thus, CVD techniques are highly preferred for this purpose.
While CVD techniques have been described with reference to many transition metals and to certain other metals (such as copper) and metalloids (such as silicon), commercial use of CVD for the most part has been confined to deposition of a few metals and metal compounds, such as silicon, tungsten, and for the co-deposition of certain III-V and II-VI compounds (denoting, respectively, a compound of a Group III metal and a Group V element, and a compound of a Group II metal and a Group VI element) such as GaAs and ZnSe.
CVD of other metals has not been extensively practiced due to a variety of reasons including poor film quality, requirement of high processing temperatures, incorporation of impurities and other defects in the deposited film, lack of suitable precursor compounds, the inability to transport vapors of the metal complex without decomposition of the vapors, and the instability of the precursors used in the deposition systems. The availability of suitable volatile and heat decomposable compounds appears to be the greatest limiting factor in the application of CVD to the production of metal containing films.
In addition to the deposition of metal films, formation of transition metal oxide coatings on substrates by CVD is known. For example, see U.S. Pat. Nos. 3,914,515 and 4,927,670 describe depositing transition metal oxide films by contacting a cyclopentadienyl metal compounds with heated substrates in the presence of an oxidizing gas.
Chapter III-2 by W. Kern et al., in "Thin Film Processes", J. L. Vossen et al., eds., Academic Press, New York (1978) at pages 258-331 provides a general discussion of CVD of thin films with specific reference to metal oxide films on pages 290-297. Such metal oxide coatings are useful in photomasks, insulators, semiconductors, high temperature superconductors, and transparent conductors, and as protective coatings for high temperature materials.
A CVD method for forming films of refractory metals would overcome some of the problems associated with the use of volatile metals such as aluminum in the manufacture of high density circuitry. Ruthenocene, (C.sub.5 H.sub.5).sub.2 Ru, has been used in the CVD of ruthenium films (D.E. Trent et al. Inorg. Chem., 3, 1057 (1964)), but this precursor has a low volatility and requires a hydrogen carrier gas to deposit a pure film. U.S. Pat. No. 4,992,305 suggests the use of substituted ruthenocenes and osmocenes such as bis(isopropylcyclopentadienyl)ruthenium and bis(isopropylcyclopentadienyl)osmium as precursors for ruthenium and osmium CVD, but no examples are given.
Osmium tetrachloride has been used for the CVD of osmium films (S. Lehwald et al., Thin Solid Films, 21, S23 (1974)), but this precursor undergoes considerable decomposition at the temperatures required for its volatilization. Thus much of the precursor is lost in nonproductive reactions. Additionally, high substrate temperatures of about 1250.degree. C. are required for the osmium deposition.
Ruthenium films have been formed by a chemical spray deposition process (J.C. Viguie et al., J. Electrochem. Soc., 122, 585 (1975)). Tris(acetylacetonate)ruthenium in butanol is converted into an aerosol spray using a hydrogen/nitrogen mixture as the carrier gas. Triruthenium dodecacarbonyl, ruthenocene, and tris(acetylacetonate)ruthenium were compared as CVD precursors in the formation of ruthenium and ruthenium oxide films by M. Green et al., in J. Electrochem. Soc., 132, 2677 (1985). None of these precursors are very volatile and thus high deposition rates were not attained.
U.S. Pat. No. 4,250,210 discloses the use of tris(acetylacetonate)ruthenium and its fluorinated derivatives in the CVD of ruthenium films. The fluorinated ligands provide greater volatility and good deposition rates are achieved when the precursor is heated to over 200.degree. C. to assist its volatilization. However, difficulties due to the stabilities of the precursors are noted and freshly prepared precursors are preferred as aged samples yield inferior coatings. Organic byproducts, presumably oligomers of the acetylacetonate ligands, with very low vapor pressures are formed and collect in the reactor. These liquid deposits represent a serious contamination problem in a production scale application of the tris(acetylacetonate)ruthenium precursors. This reference also alludes to the use of ruthenium carbonyl chloride and penta(trifluorophosphine)ruthenium as precursors for ruthenium CVD. However, these compounds are said to be unsuitable for general use because the rates of deposition of ruthenium that can be obtained are very low. Therefore, these precursors can only be used to deposit very thin coatings which exhibit poor adhesion to substrates. Additionally, ruthenium carbonyl chloride corrodes certain substrates and it is difficult to obtain a consistent product in its preparation. This lack of consistency in the product can show up as a substantially nonvolatile form of the carbonyl chloride, which decomposes before it can volatilize.
Although ruthenium thin films have been difficult to prepare, ruthenium and ruthenium oxide have been shown to have utility as electrical contact materials (R.G. Vadimsky et al., J. Electrochem. Soc., 126, 2017 (1979)). Ruthenium oxide and ruthenium metal have similar electrical conductivities and both show good environmental stability. Films of ruthenium and ruthenium oxide deposited by CVD have been proposed to be useful for contact metallizations, diffusion barriers, and gate metallizations (M.L. Green et al., J. Electrochem. Soc., 132, 2677 (1985)).
Ruthenium oxide electrodes, some prepared by CVD, have shown utility as working electrodes in nonaqueous solvents (D.R. Rolison et al., J. Electrochem. Soc., 126, 407 (1979)). Thin film multilayer cobalt/ruthenium structures, prepared by sputtering techniques, can exhibit novel magnetic properties (S.S.P. Parkin et al., Physical Review Letters, 64, 2304 (1990)). In U.S. Pat. No. 4,250,210, CVD films of ruthenium on cutting tools can increase the cutting life of the tool.
CVD of iron using iron pentacarbonyl is well known in the art. This precursor is commercially available and is very volatile. However, its high volatility makes iron pentacarbonyl unsuitable for many applications, such as its use as a dopant in semiconductor manufacture (J.A. Long et al., J. Crystal Growth, 77, 42 (1986)).
As described by C. E. Morosanu on pages 460-475 in the previously cited book, iron CVD has shown utility in preparing garnets and ferrites useful as electronics, microelectronics, microwave electronics, optoelectronics, and the like. Additional utility has been shown in the preparation of Permalloy (Ni-Fe) magnetic films, photolithographic masks, and inorganic resists for laser and electron beam lithography.
Thus, a continuing need exists for improved organometallic precursors useful for the CVD of films of iron, ruthenium and osmium.